End of Free Plan

AWS Free Tier offers 750 EC2 hours/month for free tier eligible instances. If you are running 6 instances, make sure those are free tier eligible instances and collectively usage doesn't exceed 750 hours per month.

AWS Free Tier validity is 12 months from date of activation of account. Refer AWS Free Tier Terms.

If at all, your usage exceeds this limit for a given month, then your credit card given in payment options, while setting up the account, would be charged for rest of the usage. AWS Free Tier would end automatically after 12 months but it won't stop you using AWS resources, it's just after 12 month of free tier usage of free tier eligible resources, you'll start getting charged for the usage.

Let's assume you are running 6 EC2 instances(free tier eligible) 150 hours each, then:

Your usage -> 150*6 = 900 hours

Free tier offers -> 750 hours

You'll be charged for 150(900-750) hours for that instance type. Refer EC2 Instance On Demand Pricing for exact rates for each instance type.

Please take a look at this re:Post Knowledge Center Article, this covers lot of good points to be aware of when using AWS Free Tier.

You may want to setup alert, you can consider setting up AWS budgets and configure alarm on those, which would notify you, if you cross the defined usage threshold. Follow the Well Architected Lab instructions here for setting up budget alert based on your usage.

Hope you find this information useful.

Comment here if you have additional questions, happy to help.

Abhishek